1. Field of the Invention
The present invention is directed generally to musical instruments and, more particularly, to stringed musical instruments.
2. Description of the Related Art
Many stringed instruments, such as pianos, violins, double basses, and harpsichords have sound boards that help transfer vibrational energy of the strings to vibrational energy of the air as sound. One or more sound bridges in an instrument can be used, at least in part, as an intermediary between the strings and the sound board. Unfortunately, conventional approaches for coupling the strings to a sound bridge can have undesirable consequences such as overly burdensome forces being applied to the associated sound board which degrade longevity of the instrument and awkward departures in routing of the strings causing reduced sound fidelity.
Grand pianos have traditionally been designed for over a hundred years with about 230 strings tensioned and contained in a horizontal harp shaped cast metal frame. The cast frame being located on and stiffened by a wooden subframe, which in early pianos provided the only means of containing the loads arising from the tension in the strings. Demands for more powerful sound from pianos required adopting thicker heavier strings. To ensure these vibrated at the required pitch the thicker strings needed to be at higher tension. To contain the extra stresses the cast frame was introduced.
Operation of the pianos was effected by striking the strings from below by means of a series of 85 to 97 felt covered hammers to cause the strings to vibrate, each hammer striking a single, dual, tri-chord or occasionally quadric-chord groups of strings at the same tension and of the same geometry, thus sounding the same pitch. The vibrating strings have insufficient surface area to induce sound waves in the air surrounding the instrument, thus the vibrational energy is transferred via a bridge or rectangular strip of wood attached to a sound board to cause the sound board to vibrate in sympathy with the strings and thus generate sound waves in the surrounding air.
The manner of contact between the string and the top surface of the bridge highly influences the satisfactory transfer of energy in the string to the bridge and thence to the soundboard. In traditional pianos, contact between string and bridge top is maintained by two pins driven in at an angle of about 15 degrees to the vertical. These pins are so disposed that the string changes direction by about 15 degrees as it passes first clockwise then anti-clockwise round each pin in succession. The inclination of the pin plus the change of angle of the string as it passes the pin causes a resultant downward force on the string causing it to be held against the bridge top. In consequence, the string is displaced sideways as it traverses the bridge. The amount of that displacement is called the side draught. The geometry is normally arranged so that the string line on either side of the bridge is parallel. Side draught introduces an asymmetry of the string line which is considered to be a cause of loss of clarity and purity of the piano sound. The system imposes a twisting moment on the bridge and the pins are loaded cantilevers on the bridge. The sideways forces on the pins can cause cracking of the bridge top timber. If that occurs the pin becomes loose and the note pitch may change or become false.
The first bridge pin on the sounding length side of the bridge defines the termination of the sounding length of the string. It is critical that this sounding length should be identical whichever plane the string is vibrating in or the pitch of the note will vary as the plane of vibration of the string changes. For that reason the top of the bridge is typically carved so the contact point of the string with the bridge top coincides precisely with the centre of the first bridge pin. If imprecise, this carving will permit the pitch of the string to vary according to the plane of vibration of the string. This syndrome is known as falseness. Bridge carving is a skilled and costly element of piano production.
The contact force between string and bridge top generated by the above-described disposition is inadequate to retain the string in contact with the bridge top under all conditions of operation of the piano, in particular the string can part from the bridge top when struck forcefully from below. This destroys the contact path for energy transfer from string to bridge and sound power is lost. Additional contact force is traditionally developed by arranging that each string as it traverses the bridge changes angle by about 2 to 5 degrees in the vertical plane. This change of angle produces a downwards force holding the string against the bridge top. The super position of all the forces from the many strings in a piano results in a total down bearing force of around a half ton in a typical piano. That force is contained and supported by the soundboard. The soundboard must be designed to resist this load without deterioration or collapse for the life of the piano. This implies stiffening and strengthening of the soundboard which may compromise its performance as a vibrating member generating sound waves in the surrounding air. In practice the problem of soundboard collapse and loss of down bearing is common in pianos and is the most prevalent reason for the deterioration of their power and sound quality over a period of time.
It is known that piano strings not only vibrate laterally, but also vibrate longitudinally, albeit at a far higher frequency. In the traditional concept described above for retention of the strings against the bridge top, a proportion of the length of the string lies on and is held against the bridge top. Any longitudinal vibrational movement in the string is thus effectively damped by friction, and thereby the concept causes loss of energy from the string and consequent diminution of the vibrational energy available for producing sound waves.
It is long established practice for the strings of pianos to pass from tuning pin via a fixed point normally on the frame which defines the beginning of the sounding length, and thence over the bridge with which the string is held in contact and through which the vibration energy in the string is transferred to a sounding board. In a grand piano the string is set in vibration by striking it from beneath by a felt covered hammer. The blow tends to lift the string and thus imparts a separating force between the string and the bridge cap. It is therefore necessary to provide a downwards force on the string, called down bearing, to ensure it remains in contact with the bridge cap or vibration energy transfer would be interrupted if the string parts from the bridge cap. Since the surface area of the strings is inadequate to excite significant sound waves in the air surrounding the instrument, no substantial sound would be heard unless the sound board itself receives the vibration energy input.
Two principal means are conventionally used to keep the string in firm contact with the bridge cap. The string changes angle in the vertical plane as it passes over the bridge cap. Thus it creates a downwards pressure onto the bridge cap. The change of angle is typically about 1 to 5 degrees and may be varied across the registers of the piano so that optimum contact force is developed for best sound in each register. Each string is located in the horizontal plane on the bridge by two bridge pins so positioned that the string is wrapped around each pin in the horizontal plane. The first pin defines the end of the sounding length. The two pins are so positioned that the string line is displaced sideways as it traverses the bridge. The string changes angle by about 15 degrees in the horizontal plane which is the amount of wrapping round the bridge pin. This change of angle combined with the tension in the string creates a lateral force against each bridge pin. The bridge pins are inserted in the bridge cap at about 15 to 20 degrees to the vertical at an angle which causes the string to tend to slide down the pin and thus press against the bridge cap. This develops an additional pressure contact between string and bridge cap without, in this case, causing a down bearing load on the soundboard.
This traditional system has consequent disadvantages that affect the performance and durability of the piano. The down bearing force resulting from the change of angle of the string in the vertical plane as it passes over the bridge from about 200 strings in a typical piano amounts to approximately a half ton. That force is applied constantly throughout the life of the piano to the sound board. The sound board must therefore be so designed to withstand the load for the life of the piano. This entails creating a sound board stronger and stiffer than would be ideal for optimum acoustic performance. If the designer creates a thin light sound board he can enhance the piano acoustic performance at the expense of a shorter instrument life. As the crowning of the soundboard sinks with time due to this load, the contact force between strings and sound board decreases and the efficiency of the transfer of vibration energy from strings to sound board becomes compromised. The piano as a consequence loses its acoustic performance. In extreme cases the string may part from the bridge cap when the hammer strikes and then buzzing sounds develop.
The downwards loads from the strings to the sound board may also compromise the freedom of the soundboard to respond to the vibrational energy from the strings and this may result in degraded sound volume and quality, the duration and the harmonic content of the sound developed is spoiled. It is for example well established that down bearing load on the bass bridge of a piano will suppress the performance of the middle treble range of the registers because the bass bridge is located near the soundboard zone that needs to respond to the middle treble register frequencies.
Because the strings are struck from underneath it is found that with forceful playing they tend to slide up the inclination of the bridge pins and be held away from firm contact with the bridge cap by friction between the pin and the string. Piano technicians will need to tap them down again after vigorous playing or the sound performance will suffer.
The bridge pins are inserted in the bridge by drilling inclined pilot holes of controlled depth. It is critically important that each pin is firmly located and the hole into which it is fitted is not bell mouthed which would allow the pin to flex at its upper end. An insecure pin may result in a badly defined length of the sounding portion of the string. In that case each string may produce a varying frequency known as a false note.
The two sets of bridge pins are mounted about 20 mm apart. Because of the wrap angle round the pins the string line is displaced sideways where the string traverses the bridge. In consequence of this asymmetry it is found that a piano string that is initially excited to vibrate in the vertical plane will begin to develop a component of vibration in the horizontal plane. The plane of vibration rotates with time. The quality and volume of sound when the string is vibrating horizontally is degraded and the overall sound quality of the instrument may be compromised by a weak or variable note quality.
Where the string traverses the bridge cap it lies in contact with the bridge cap surface. When the string is excited by striking with the hammer, most of the vibration energy is in transverse movement, but a proportion also appears as longitudinal vibration. Longitudinal rubbing between the string and the bridge cap results in loss of vibration energy in the string and reduction in the efficiency of use of the available vibrational energy produced in the string.